A strip-lined painting consists of four lap joints. In terms of mechanics, a painting suitable for strip-lining is a relatively flexible material adhered with an adhesive of lower shear stiffness (e.g. Beva 371), to another flexible material (linen/polyester sailcloth/polyester monofilament). The main advantage of this design is its simplicity. Its main disadvantage is that when in tension, varying shear and tensile stresses are produced in the adhesive and the adherends. Samples were prepared with pre-primed linen (oil and acrylic) to represent paintings of different stiffness, using an extreme case of epoxy (high shear stiffness) and Beva 371 (low shear stiffness). The samples were subjected to a biaxial tension between 30 N–70 N, at constant temperature and RH. Electronic Speckle Pattern Interferometry (ESPI) was used to obtain the 3D full field strain and enabled areas of strain concentrations to be identified and profiled across the strip lining. The out-of-plane data quantified the degree of bending of the sample resulting from the uneven nature of the lap joint. This research established under what conditions adhesive lap-joint theory is valid for application to paintings conservation; it predicted theoretically and confirmed experimentally that the peak shear and tensile strains occur at the two ends of overlap and into the picture plane. This concentration of strains is due to the differential strain of the adherends and bending under tension of the joint.